Vital signs such as heart rate, oxygen saturation (SpO2), temperature, and blood pressure constitute basic assessments of patients' health in clinical environment. These vital signs are either directly measured or indirectly extracted from physiological waveforms such as electrocardiogram (ECG) and photoplethysmogram (PPG). While these vital signs are continuously monitored in an in-patient ward, they are highly under-sampled for out-patients as they are mainly obtained at clinics or hospitals not at home or the workplace. Recently, several wearable vital sign monitors have been introduced with the advancement of wearable sensor nodes as well as the reduction of their form factor. At the same time, the battery operated wearable monitor significantly constrains a power budget because the power consumption of the monitor mainly determines the maximum operating time without replacing the battery.